Evidence for cellular control in the synthesis of acetoin or α-ketoisovaleric acid by microorganisms

Commercial α-acetolactate at pH 4.5 decarboxylated nonenzymatically (5 to 8%/h) to acetoin (69%) and diacetyl (31%), and an extract of Streptococcus diacetilactis 18-16 produced α-acetolactate (in addition to acetoin and diacetyl) from pyruvate in the presence of TPP and MgSO 4 . Nevertheless, α-acetolactate was not dispersed into media by any of four microorganisms (S. diacetilactis, strains 18-16 and DRC1, Saccharomyces cerevisiae 299, and Lactobacillus casei 393) that produced diacetyl and acetoin or by one (Serratia marcescens) that produced only acetoin. Lactobacillus casei and S. diacetilactis 18-16 produced unknown compounds that falsely indicated the presence of α-acetolactate when tests were made without separating acetoin and diacetyl from other components of the spent media. The production of acetoin by S. diacetilactis 18-16 was not inhibited by valine, the acetoin produced by this organism was optically active (+101.0°), and a cell-free extract of S. marcescens did not produce diacetyl while producing a large amount of acetoin. Data support the conclusion that the conversion of pyruvate to acetoin by some microorganisms and to α-ketoisovaleric acid by others is enzymatic and under cellular control, resulting in the synthesis of only steady-state amounts of enzymatically bound α-acetolactate in each of the pathways.